The configuration of a conventional coordinate input device having a cordless position pointing device 200 is best shown in FIG. 3. Such a coordinate input device employs an electromagnetic transfer method, and has a plurality of sensor coils provided side by side in the direction of the X axis, as well as in the direction of the Y axis, of a coordinate input surface. An alternating magnetic field is transmitted from the sensor coils by feeding current to the sensor coils in a sending mode. When the transmitted alternating magnetic field excites a coil or a resonant circuit in the position pointing device 200, the position pointing device 200 transmits a response to the alternating magnetic field. When the sensor coils stop sending the alternating magnetic field, they are in a receiving mode, and receive a response to the alternating magnetic field. A coordinate of the position pointing device 200 is then calculated by analyzing signal strength distribution of received signals in each of the sensor coils.
High-frequency signals generated by a high-frequency signal generator 121 are transmitted to the sensor coils in the sending mode. The coordinate input device has coil switching means for selecting each of the sensor coils in order to transmit the high-frequency signals, or to receive receiver signals, and has transmission/reception switching means for switching between the sending mode and the receiving mode in each of the sensor coils. The coil switching means may be separately provided from the transmission/reception switching means, or it may serve as the transmission/reception switching means. Generally, switching means such as an analog switch is used. The receiver signals are passed to a signal analysis section through a receiving circuit 145. The electromagnetic transfer method is described in detail in Japanese Examined Patent Application Publication No. 2-53805, Japanese Unexamined Patent Application Publication No. 8-286814, and Japanese Unexamined Patent Application Publication No. 8-249105.
A sensor coil group and the receiving circuit of FIG. 3 is best shown in FIG. 4A. A transmitting circuit 220 includes the high-frequency signal generator 121, a coil switching section 240, a sensor coil group 260, and the receiving circuit 145. In the transmitting circuit 220, a separation circuit 150 receives a signal Vin generated by the high-frequency signal generator 121. The separation circuit 150 separates the signal Vin into a half-wave signal to which positive bias voltage is applied (the top waveform in FIG. 4B) and a half-wave signal to which negative bias voltage is applied (the bottom waveform in FIG. 4B). The negative signal is input to a positive input terminal IN18 of an operational amplifier 120 (hereinafter referred to as an op-amp), and the positive signal is input to a positive input terminal IN20 of an op-amp 122. The op-amps 120 and 122 control the voltage corresponding to signal voltages input to the input terminals IN18 and IN20, respectively, such that the voltage is applied to the lower end of a resistor R30 and the upper end of a resistor R32, respectively. A current having a potential difference divided by the corresponding resistance of the resistor R30 or R32 flows through the resistor R30 or R32. Each of the op-amps 120 and 122 generates a high-frequency current corresponding to input high-frequency signals. The generated high-frequency current is combined into a full-wave signal through a driver transistor 124 or 126 that is transmitted to the coil switching section 240.
The coil switching section 240 includes pairs of sending analog switches 128, 130 . . . 132 and corresponding receiving analog switches 134, 136 . . . 138. The pairs are connected to the sensor coils 140, 142 . . . 144 included in the sensor coil group 260, respectively. To select one of the sensor coils 140, 142 . . . 144 for the sending mode, the sending analog switch connected to the selected sensor coil is turned on and the remaining analog switches are turned off. To select one of the sensor coils 140, 142 . . . 144 for the receiving mode, the receiving analog switch connected to the selected sensor coil is turned on and the remaining analog switches are turned off. Sequentially turning on/off these analog switches makes it possible to sequentially transmit the high-frequency signals to the sensor coils 140, 142 . . . 144, and to sequentially obtain received signals as the response to the alternating magnetic field. The received signals as the response to the alternating magnetic field are voltage signals based on induced voltage generated in the sensor coils 140, 142 . . . 144 owing to an electromagnetic interaction with the response to the alternating magnetic field. The received signals from the sensor coils 140, 142 . . . 144 are output to an output terminal OUT14 through the receiving circuit 145. The output signals are passed to the signal analysis section where coordinate calculation is performed.
In the conventional coordinate input device described above, a pair of a transmitting analog switches and a receiving analog switch are connected to each of the sensor coils 140, 142 . . . 144, and the high-frequency signals are transmitted with these analog switches. Accordingly, power loss results due to voltage loss by on-resistance of the analog switches. Thus, the transmitted current undesirably decreases. Under low power supply voltage, insufficient transmitted current may result in serious problems.
In addition, in the conventional coordinate input device described above, stray capacitance 160 and 162 may increase due to connection of a plurality of sending analog switches and a plurality of receiving analog switches. The stray capacitance 160 of the sending analog switches decreases output impedance when transmitting the high-frequency signals, thereby causing poor constant-current characteristics. Furthermore, resonance between the stray capacitance in the sending analog switches and the sensor coils is triggered, which decreases resonant frequency. Therefore, it becomes difficult to maintain the stability in the high-frequency signal generator in the transmitting circuit.